1. Field of the Invention
This invention relates to the introduction of fluids into subterranean formations, more particularly to scale removal in the formations and stabilization of formation fine particles against movement.
2. Description of the Art
A long-recognized problem in producing fluids such as water and/or oil from subterranean formations is scale deposition. One very common scale is based upon calcium carbonate, which precipitates from pressurized aqueous fluids containing calcium ions and bicarbonate ions; as fluid pressure is reduced during production of the fluids, carbon dioxide is released and calcium carbonate precipitates, according to the equation: EQU Ca.sup.+2 +2HCO.sub.3.sup.- .fwdarw.CaCO.sub.3 +CO.sub.2 +H.sub.2 O
Another common scale is alkaline earth metal sulfates formed during production of fluids which contain high concentrations of dissolved alkaline earth and sulfate ions. Other scales can be formed by corrosion of equipment in the well, particularly steel tubing and casing, which yield various compounds, including iron and other metal sulfides, oxides, and the like.
Scale materials tend to form in a formation very close to a producing well, normally only a few inches into the formation, and in the tubing, casing, and other equipment of the well. Scale deposits in pores of the producing formation, perforations in well casing, and other openings necessary for fluid flow, sometimes causing extremely rapid declines in that flow.
It is customary to deal with scale deposits by techniques such as acidizing, wherein an acid capable of dissolving the scale is injected into areas suspected to contain scales. Acidizing is effective in many cases, but has concomitant disadvantages, not the least of which is rapid corrosion of metals contacted by the acids.
Case, in U.S. Pat. No. 2,877,848, and C. M. Shaughnessy et al., "EDTA Removes Formation Damage at Prudhoe Bay," Journal of Petroleum Technology, October 1983, pages 1783-1791, describe methods for removing scale from formations and wellbores, by pumping aqueous solutions of ethylenediaminetetraacetic acid salts into the wells and formations.
Another very common phenomenon tending to reduce fluid flow in formations which contain finely divided particulate matter is the migration of such particles into constrictions of the formation pores. Particularly when the particles are silt-sized or smaller, even relatively low fluid flow rates through a formation can cause movement of the particles. Also, certain types of water-sensitive clays which can be present in a formation as fine particles, e.g., montmorillonite, can swell and decrease the permeability of the formation if contacted by water.
Various treatments have been proposed to minimize damage to formation permeability by fine particles. Such treatments have included injecting zirconium oxychloride to stabilize clays, converting clays from the swelling sodium or lithium forms to a lesser swelling cation form, and injecting various amine or hydroxyaluminum compositions.